1. Field of the Disclosure
This disclosure generally relates to apparatus and methods for filtering a biological fluid, such as (but not limited to) the removal of leukocytes from whole blood or a blood component. More particularly, the disclosure relates to apparatus and methods for removing air from a fluid container having an amount of filtered fluid.
2. Description of Related Art
Prior to transfusion of blood or a blood component into a recipient, it is common to filter the blood to remove leukocytes. This process is commonly referred to as leukoreduction. It may be desirable to remove leukocytes from blood or a blood component prior to transfusion because they can trigger a broad range of adverse reactions in a recipient, ranging from minor effects, such as chills, to more serious effects, such as the transmission of cytomegalovirus, which can be fatal to recipients with weakened immune systems.
Commonly, leukoreduction involves the transfer of blood or a blood component from a pre-filter fluid container to a post-filter fluid container through a tubing line having a leukoreduction filter. The filter typically includes a quantity of air that is pushed out of the filter upon priming the same during a filtration application. It is desired to prevent this air from moving into the post-filter container and remaining there, because such air aggregation can prevent complete filtration of the blood, as will be described in greater detail herein, and decrease the quality and storability of the filtered blood. Even when the blood or blood component is used a short time after filtration, there is a general preference among users to have as little air in the post-filter container as possible.
Known approaches to air management include filtration systems that are vented to the atmosphere or a gas container and those incorporating a bypass line. For example, U.S. Pat. No. 5,863,436 to Matkovich, which is hereby incorporated herein by reference, describes several leukoreduction systems incorporating one or more air vents. One commercial system according to the description of Matkovich is the Pall SAVE™ system, which is incorporated into the Leukotrap® WB Filtration System from Pall Corporation of Glen Cove, N.Y. The Leukotrap® WB Filtration System comprises a pre-filter container connected to a post-filter container by a filter line having a leukoreduction filter. A pre-filter air vent is associated with the filter line between the pre-filter container and the filter, while a post-filter air vent is associated with the filter line between the filter and the post-filter container. In use, the pre-filter container is hung above the post-filter container and a cannula of the pre-filter container is broken to allow fluid flow into the filter line. The fluid is prevented from flowing into the pre-filter air vent by a removable cap, so it instead flows into the filter. The filter is allowed to prime, with air exiting the system through the post-filter air vent. When the filter is fully primed, a cannula between the post-filter vent and the post-filter container is broken to allow fluid and air to flow into the post-filter container. Due to pressure differentials in the system, the filtration process ceases prior to complete filtration of all the fluid, i.e., with an amount of fluid remaining in the filter. At that time, the cap on the pre-filter air vent is removed to allow a gas to enter the filter line and purge any remaining fluid from the inlet side of the filter.
One problem with systems according to the foregoing description is that no means are provided to remove air from the post-filter container, either during or after filtration. While the post-filter vent removes the air that is purged from the filter, gas may be initially present in the system at other locations, such as in the containers or the tubing, as a result of the manufacturing process. This gas is pushed into the post-filter container during filtration and can lead to the aforementioned diminished performance and quality concerns if not removed during or after filtration.
In response to the foregoing problem, leukoreduction systems incorporating bypass lines allow removal of air and other gases from the post-filter container during and/or after filtration. Several examples of known leukoreduction systems with bypass lines are described in U.S. Pat. No. 6,358,420 to Blickhan et al., which is hereby incorporated herein by reference. In one system, a pre-filter container is connected to a post-filter container by a filter line having a leukoreduction filter. Tubing comprising a bypass line is connected to the filter line at opposite sides of the filter, thereby allowing for fluid communication between the containers along a path that bypasses the filter. The bypass line is provided with a one-way valve, typically a check valve, which only allows air and fluid flow toward the pre-filter container from the post-filter container. In use, the pre-filter container is hung above the post-filter container and a cannula of the pre-filter container is broken to allow fluid flow into the filter line. The fluid is prevented from flowing through the bypass line and into the post-filter container by the one-way valve. The fluid flows through the filter and into the post-filter container, along with an amount of air. Due to pressure differentials in the system, the filtration process ceases prior to complete filtration of the fluid, i.e., with an amount of fluid remaining in the filter. At that time, a slide clamp is placed on the filter line, between the filter and the post-filter container, and the post-filter container is squeezed to force air through the bypass line and toward the pre-filter container. Squeezing the post-filter container to remove air and other gases is sometimes referred to as “burping” the container. When the post-filter container has been “burped,” the clamp is removed from the filter line and the filter is allowed to more completely drain.
According to another leukoreduction system described in Blickhan et al., one end of the bypass line is connected to the filter line at a position between the pre-filter container and the filter, while the other end is connected directly to the post-filter container. This system operates similarly to the previously described system of Blickhan et al. to filter blood or a blood component and remove air from the post-filter container.
While systems incorporating bypass lines represent improvements over the systems of Matkovich in terms of air removal from the post-filter container, the need to manually “burp” the container to remove air may be problematic. In particular, the amount of air removal is directly dependent on the skill of the user, which can potentially lead to insufficient or incomplete air removal.
A more recent approach to eliminating the manual “burping” step is to allow for automatic “burping” of the post-filter container. Several such systems are described in U.S. Pat. No. 6,171,493 to Zia et al., which is hereby incorporated herein by reference. Rather than connecting the bypass line to one or more sections of the filter line, one end of the bypass line is directly connected to the pre-filter container and the other end of the bypass line is directly connected to the post-filter container. The pre-filter container is hung above the post-filter container and, in one embodiment, a loop portion of the filter line is elevated above the fluid level in the pre-filter container to prevent fluid from flowing through the bypass line and into the post-filter container. A clamp on the filter line is opened to allow fluid flow through the filter line and the filter. Air in the filter is pushed into the post-filter container by the blood and begins to accumulate therein and/or to leak from the post-filter container into the bypass line. When the pressure in the post-filter container reaches a sufficient level and the pressure in the pre-filter container decreases sufficiently (typically to a vacuum state), some of the air moves up the bypass line, through the loop portion, and into the pre-filter container. The return of air to the pre-filter container increases the pressure above the filter and assists in more completely draining any remaining fluid from the filter.
In theory, the “burping” system of Zia et al. improves on previously known systems by automatically removing air from the post-filter container, without requiring a manual “burping” operation. However, the efficiency of the Zia et al. system is contingent on the pressure differential between the post-filter container and the pre-filter container. Optimal filtration results are achieved when pressure in the post-filter container is maximized. If only a small amount of fluid is to be filtered, then the post-filter container will remain relatively empty and the pressure developed therein will not be sufficient to re-circulate the air to the pre-filter container. In such situations, the post-filter container must be manually squeezed to remove air, thereby representing a failure of the intended automatic “burping” feature.
Therefore, there remains a need for apparatus and methods for more efficiently removing air from a post-filter container, especially during filtration of a smaller amount of fluid.